1. Field of the Invention
The present invention relates to the recovery of metal values from ores and, specifically to a process and composition for agglomerating ore fines to enhance the economic recovery of metals in a heap leaching recovery process.
2. Description of the Prior Art
The recovery of precious metal values by cyanide solutions, and leaching by virtue of the standard heap leach process are known in the art. Such process are described, for example, in Landenan et al. U.S. Pat. No. 3,777,004, issued Dec. 4, 1973.
It is well recognized that the state of Nevada has extensive ore deposits of gold and silver. In these deposits, as well as in deposits in other states and localities, there has been found to occur an appreciable amount of fines, i.e., minus 40 mesh or finer, in the ore material. Such fines inhibit the degree of success that has previously been achieved in connection with recovery of precious metal values from such ores. A great deal of study has been conducted in connection with the character of such fines, resembling fine-particle clays of bentonitic type, and the effect of such fines in the presence of ores undergoing treatment for precious metal values.
In the prior art, heap leach piles and leaching solutions have been used with varying degrees of success. Many such processes have not been economically successful since the fine plasterings over rocks and interstices of heap leach piles have prevented the advantageous recovery of pregnant solutions or have allowed leaching fluids to percolate therethrough. In order to economically heap leach gold and silver ores, it has thus been found necessary to agglomerate the ores before leaching can begin, using some type of binder. The agglomeration prevents migration of ore fines, swelling of clays, and collapse of the heap. Migration of fines and swelling of clays can blind part of the heap making efficient leaching impossible. Collapse of the heap can seal-up large areas to the leaching solution and channel the solution around areas where it is needed. At the present time, high calcium lime and Portland cement are used as binders. Both of these binders have the advantage of maintaining the pH of the leaching solution in the range from about 10-12 which is necessary for leaching with a cyanide solution.
Lime, Ca(OH).sub.2, prevents the swelling of clay by replacing monovalent cations, Na+ and K+, with the divalent cation, Ca++. The presence of divalent cations prevents significant swelling of clays in the recovery process. Also, lime reacts with silica and alumina in the clays and ore fines giving calcium silicate and aluminates, hydrates binding the agglomerates together, thus preventing migration of fines and heap collapse.
In Portland cement, calcium silicates and aluminates are already intimately mixed so that the hydrates form rapidly, giving good strength to the agglomerate. If the ore is of low clay content or the clay has a low pozzolinic activity, i.e., the low availability of silicates and aluminates, I have found that Portland cement will make a higher strength binder than lime. Portland cement has low free calcium hydroxide content, however, and accordingly is not as efficient as lime in preventing swelling of clays in high clay content ores.
The present invention has as its object the provision of an economical recovery technique for precious metal values from metal ores through the use of an improved binder composition for agglomerating the ore fines. The technique has particular applicability to the recovery of gold and silver